Wrong filing of gstr1

Hello, 

Is there any way of rectifying error in filing wrong GSTR1?

Our firm had received an advance for the month on July. We mistakenly forgot to fill the details of the same in GSTR3B and then also did not fill it in GSTR1. Therefore, we paid less tax in July than what we are liable to actually pay. Could you please advice how to rectify this error in GSTR1 now.